1. Field of the Invention
The present invention relates to a multilayer optical information recording medium having a plurality of information recording layers where information is recorded and an information recording apparatus for recording information onto the multilayer optical information recording medium.
2. Related Background Art
In recent years, mass-storage optical disks, typically CD or DVD, have come into wide use. Additionally, to further improve recording densities of optical disks according to demand for recording long-time video data or the like, a multilayer optical disk having two or more information recording layers is under development. In addition, if each information recording layer is formed by a phase change recording layer, it becomes possible to realize a multilayer optical disk capable of recording information in addition to reproducing information.
The phase change recording layer of the multilayer optical disk, however, is put in a crystalline state initially before recording, while it is put in an amorphous state after the recording. In general, an unrecorded area having the phase change recording layer in the crystalline state differs from a recorded area having the phase change recording layer in the amorphous state in a reflectance and a transmittance. Assuming such a condition that an information recording layer, which is an object of recording, is irradiated with a laser beam in recording onto a recordable two-layer optical disk and that its reflected light is received, the recording on the information recording layer near the laser source is affected by a reflectance of an underlying information recording layer. Furthermore, during recording onto an information recording layer far from the laser beam emitting side, the recording is affected by a reflectance or a transmittance of an overlying information recording layer. Unless a uniform distribution is observed in the transmittance or the reflectance of each information recording layer, an intensity distribution may be uniformless in the optical beam of an optical detector.
Particularly, in performing a tracking servo in an information recording apparatus for recording onto the multilayer optical disk, a variation of the transmittance or the reflectance remarkably affects the tracking servo. In other words, if the recorded area is mixed with the unrecorded area in the information recording layer, components caused by the variation of the transmittance or the reflectance are superposed on the light intensity distribution in the optical detector. Additionally, if a laser beam is transmitted or reflected in the vicinity of a boundary portion between the recorded area and the unrecorded area, it may cause an offset in a tracking error in the information recording layer which is an object of recording. The offset occurs in the tracking error since the effect of the boundary portion causes laser power to be asymmetric between one side and the other side in the disk radial direction even if a track is traced accurately in detecting a tracking error in the push-pull method generally used for the information recording apparatus. As set forth hereinabove, there has been such a problem that a performance of the tracking servo is unsuccessfully secured due to the offset of the tracking error caused by a recording condition of each information recording layer when information is recorded onto a recordable multilayer optical disk.